1. Field of the Invention
The present invention relates to a method of forming dichromatic copy images free of fog and mingling of colors.
2. Description of the Prior Art
To meet the need for diverse modes of processing information in recent years, copying machines have been developed for making copies of composite images by successively or simultaneously forming on the surface of an electrostatic latent image bearing member a first latent image and a second latent image having an opposite polarity to the first image, or having the same polarity as the first image but different therefrom in potential, developing the first and second latent images thus formed as a composite image to obtain a toner image, and transferring the toner image onto the surface of a copy material. Moreover, copying machines capable of producing dichromatic copy images from a dichromatic original, for example from an original bearing a red colored images and black colored images have also been proposed.
Although it is quite apparent in a case of obtaining dichromatic copy images from a dichromatic original, there is frequent necessity also for the above described composite images to develop the first and second latent images with different colors of toner respectively for the purpose of editing and discrimination. This dichromatic development is normally performed with use of two developing devices wherein a latent image corresponding to an image portion of first color is initially developed by a toner of first color and then developing a latent image corresponding to an image portion of second color with a toner of second color. However, when the magnetic brush development method is utilized in particular, a small quantity of the toner of first color developed by the first developing device is scraped off during the development by the second developing device so that the toner of first color mixes into the second developing device which accommodates the toner of second color. In this case, the first and second developing devices generally employ two component developer of a mixture of toner and carrier triboelectrically chargeable to opposite polarities. Accordingly, when the toner of first color intrudes into and is mixed into the second developing device, it charges triboelectrically with the carriers thereby making the charging characteristic unstable and in consequence, the charging polarity of the toner of second color changes to cause mingling of colors. As a result, reproduction of image with mingling of colors cannot be avoided but also the scattering of toner and fogs are bound to occur so that it is quite difficlut to obtain fine dichromatic images.
For this purpose, there has been proposed in Japanese Unexamined Laid-open Patent Application No. SHO 56-55971 a dichromatic developing method wherein first and second magnetic developing devices are provided adjacent one another and the toner of first color is prevented from mixing into the second developing device by making the magnetic force of the second developing device stronger than the first. In Published Unexamined Japanese Patent Application No. SHO 56-130773, a device for separating toner mixed into the second developing device is provided to prevent mingling of colors. However, these measures taken to prevent mingling of colors are all structural and cannot avoid making the developing device complicated.